There are numerous areas of the world where electrical energy is not distributed and available to the public. In these areas electronic equipment such as radio receivers, tape recorders and players, computers, and even small light sources must operate from local power supplies, usually batteries. Primary batteries are common, but must be discarded when exhausted. Secondary batteries are also known, and can be recharged after use if a suitale source is available to which they can be connected: the efficiency of such batteries decreases as their temperature falls. It is also known that solar cells may be connected to supply charging energy for suitable secondary batteries when the cells are positioned for impingement by sunlight: the efficiency of such cells decreases as their temperature rises.